1. Field of the Invention
The invention relates to a rear projection screen comprising an optically transparent plate having a first surface which is provided with a structure of juxtaposed cylindrical lenses extending in a first direction and, arranged opposite thereto, a second surface which is intended to be directed towards an audience space and is provided with first, light-absorbing strips also extending in the first direction, said plate comprising diffusing means on the second surface.
The invention also relates to an image projection device comprising such a rear projection screen and to a method of manufacturing such a screen.
2. Discussion of the Related Art
When projecting an image in an audience space, the aim is to give the image formed on a projection screen a homogeneous and large brightness in a limited part of the audience space, viz. only in the space where the audience is present. In this way the available radiation energy can be optimally utilized for the image observed by the audience. The part where the audience is present is, however, a relatively limited part and has different dimensions in the vertical and horizontal directions. In the horizontal direction the viewing angle should be as large as possible, whereas the vertical viewing angle is generally much smaller. To realise the different viewing angles, the image projection screen may be provided with means which ensure a horizontal spread and means which ensure a vertical spread of the light.
A rear projection screen of the type described in the opening paragraph is known from U.S. Pat. No. 3,832,032. The projection screen described in this Patent has a structure of juxtaposed cylindrical lenses extending in the vertical direction on the surface facing the projector. The cylindrical lenses focus the signal light from the projector into focal lines on the surface directed towards the audience space. The surface directed towards the audience space is provided with first strips of light-absorbing material outside the focal lines formed by the lens structure and ensures the spread of light in the horizontal direction. Since the signal light is concentrated by the cylindrical lenses in the spaces between the light-absorbing strips, this light is thus substantially not subjected to absorption by the strips, while a large part of the ambient light in the audience space is absorbed by these strips and is thus suppressed. An image having a considerably high contrast can be realised in this way.
The surface directed towards the audience space is roughened in the areas between the strips of light-absorbing material in order to diffuse the incident light from the lens structure into the audience space. This is realised by providing a large number of scratches or microgrooves parallel to each other. Dependent on the orientation of the longitudinal axes of the grooves, there is diffusion in the horizontal and the vertical direction so that the spread in the horizontal direction is ensured by the combination of the lens structure and the grooves, while the spread in the vertical direction is ensured by the grooves only. However, U.S. Pat. No. 3,832,032 explicitly prefers orientation of the grooves in such a way that light is predominantly diffused in the direction perpendicular to the direction in which the lens structure diffuses light, notably in the vertical direction.
A drawback of the known projection screen is that two separate elements are required for the desired spread of light in the horizontal and vertical directions, which elements must diffuse light through different angles if a screen is desired whose diffusion has a given asymmetry
Since the grooves are provided by means of sandblasting in the known screen, it is further relatively difficult to accurately adjust the diffusion angle of the grooves. In fact, the diffusion angle is determined by the orientation of the plate to be sandblasted with respect to the jet with which sandblasting is carried out. Moreover, the vertical direction, i.e. the direction in which the grooves diffuse, is exactly that direction in which it is most important to control the diffusion so as to obtain an optimum radiation distribution in the audience space.
It is to be noted that diffusers in the form of spherical particles are known which ensure diffusion of light simultaneously in the horizontal and vertical directions. However, this has the drawback that the diffusion is circularly symmetrical so that extra measures will still have to be taken when a different viewing angle in the vertical and horizontal direction is desired.